Today, there are several data buses or field buses that are intended to distribute control signals between a central unit, also called a “master”, and several nodes, also called “slaves”. A number of these field buses make use of only one conductor pair to distribute both control signals, in the shape of data pulses, and electrical energy, for operating the nodes and their peripheral units. One typical example is presented in the U.S. Pat. No. 4,139,737. Here, a pulse train of square pulses is superimposed on a supply voltage.
In prior art field buses that transfers data and energy on the same conductor pair, different pulse trains are sent over the bus to transfer data. This data is interpreted as addresses to different nodes, as identities of different information packages, as instructions or control signals or as measurement values or status information from nodes. In most prior art pulse trains, each pulse or each time slot is used to transfer one data bit, i.e. the information is normally in a binary form. During the pulse or time slot, the voltage or current on the bus is either high or low, representing either a data bit with the value 1 or 0, respectively. Alternatively, pulses having different duration can represent different digital values.
The U.S. Pat. No. 4,477,896 is another typical example of a system utilising transmission of data and power over the same conductor pair. Information from a base unit to nodes is sent as time divided voltage pulses. Information from nodes to the base unit is instead represented as time divided multiplexed current pulses. The voltage signals are preferably superimposed on a DC potential, which is used as a power source for the nodes.
Prior art systems of this kind are favourable in many respects, e.g. since they reduce the need for multiple parallel control signal wiring. However, a number of disadvantages still remains. One problem is that the pulse duration occupies a relative large portion of the total time. During the pulses, the ability of the bus to transfer energy, i.e. the efficiency in powering of the nodes, is detrimentally affected. The powering arrangement therefore has to be deliberately overdimensioned to compensate for this lower efficiency.
In the published patent application DE 38 28 271, a telegram transmission system is disclosed using a pulse position modulation. A time difference between short square pulses is interpreted in terms of data of typically more than one bit. Slave units are also powered by the communication bus.
Another problem is connected with the use of field buses in environments that are sensitive for electromagnetic radiation. The use of frequently occurring substantially square-shaped pulses gives rise to relatively large amounts of electromagnetic radiation. Depending on the used pulse frequency, the radiated energy appears in different frequency ranges. Since the square pulses furthermore involves a relative large portion of high-frequency overtones, radio frequency disturbances are to be expected in many cases. In environments sensitive to such disturbances, the emission of electromagnetic energy from the data bus may cause large problems.
The digital data is in prior art systems typically transmitted as relative long sequences of pulses, where the voltage value at a certain time has a certain meaning. The emission of the pulses is typically controlled by a clock device in the sending unit. The interpretation in the receiving end is analogously strongly dependent on that the receiving unit has basically the same definition of time. The receiving unit thus has to have a clock device with a stability that is accurate enough to ensure a correct interpretation of the pulses during the entire pulse sequence. However, accurate clock devices are relatively expensive and this causes a problem when very simple and inexpensive nodes are to be connected.
In DE 199 46 776, simultaneous bi-directional communication between two units is disclosed. The data to be transferred is modulated as binary data. Amplitude modulated voltage pulses are used for transferring binary data in one direction, and amplitude modulated current pulses are used for transferring binary data in the opposite direction. However, the disclosure is limited to two communicating units, and can not in an obvious manner be modified to multi-node systems.
A typical communication between the central unit and the more than one remote nodes takes place at the command of the central unit. In a typical signalling procedure, the central unit first sends out an address to a node to be contacted. Then, an amount of data may be sent from the central unit to the addressed node. The node may then respond, e.g. by a confirmation. Only the addressed node has this possibility, since there cannot exist any simultaneous signalling from more than one node at a time. The nodes have in a typical case no or at least small possibilities to initiate a communication with the central unit at its own initiative. This may cause problems when emergency situations occur. If an emergency situation occurs at a node, the node has to wait for the central unit to address it, before any message of the emergency situation can be transferred. If the central unit serves a large number of nodes, it may take a long time between each successive addressing.